In producing chemical wood pulps, it has become the practice to use cooking liquors containing various cooking chemicals for liberating the pulp fibers. The so-called kraft or sulphate pulp is produced by cooking the raw chipped wood in a liquor wherein materials such as sodium hydroxide and sodium sulfide serve as the essential fiber liberating chemicals. The so-called soda pulp derives its name from the caustic soda-containing cooking liquor which is produced, namely a liquor containing principally caustic soda as the active pulping chemical. There are modifications of these processes based on the use of liquors containing caustic soda and sodium sulphite or containing caustic soda, and sodium sulphide. All of these processes are, however, performed similarly with respect to the cooking being effected with an amount of liquor over a period of time requiring the addition of heat to maintain the process at the proper cooking temperature, approximately 170.degree. C.
Two basic processes have been used for performing the chip cooking. The first is batch cooking in which the chips are placed in a digester, liquor is added, the temperature and pressure are raised and the "batch" is maintained at the elevated temperature and pressure to reach the desired stage of delignification. The digester is then emptied, and a subsequent fill is started for another batch. In continuous digesting, the second basic process, a chip column continuously moves through the digester with hot liquor circulating therethrough. Process conditions are controlled such that the desired stage of delignification has occurred when the chips flow out of the digester.
Various advances have been made in batch cooking processes utilizing spent liquor or black liquor in transferring heat to the chips, but effective heat conserving processes for continuous cooking have not been developed to an advanced stage for attaining maximum heat energy conservation.
In conventional continuous digesters, the spent liquor is allowed to flash and steam is generated. The steam is normally utilized to pre-steam the chips and to generate hot water. The heat and cooking chemicals could be utilized more efficiently if the spent liquors were used to preheat and precondition the chips and to preheat the cooking chemicals such as white liquor in kraft processes which enters the process.